The present invention relates generally to the housing for a circuit breakers and in particular to a housing which includes a base having improved structural characteristics and a subbase which allows easy assembly of the breaker.
A conventional circuit breaker comprises several components including source and load side terminals, a line strap, a movable load contact arm, a latch mechanism which is used to manually open an close the breaker and an electromagnetic tripping device which operates in response to an overcurrent flowing through the load contact arm and line strap to trip the latch and open the breaker. For circuit breakers which protect three-phase circuits, all of these components except for the latch are duplicated three times.
The assembly of a conventional circuit breaker may be a complex task because some of these components overlay other components in the breaker housing. Because high currents are involved, each of the component parts of the circuit breaker is typically mounted in the breaker housing with tight tolerances. For example, in many breakers it is desirable for the principal current carrying components, the load contact arm and the line strap, to be parallel and in close proximity along at least a portion of their length and yet insulated from each other. This construction enables these breaker components to generate the strong magnetic repulsive forces that are used to "blow-open" the connection between the load and line contacts during a large overcurrent condition. In many existing breakers this structure is achieved by applying insulation directly over each line strap and then assembling the load contact arm directly over the insulation.
The assembly requirements of a circuit breaker contribute directly to its cost. Accordingly, it is desirable to design a circuit breaker to simplify its assembly as much as possible.
One way in which assembly may be simplified is to provide more open space in the circuit breaker housing. A relatively open housing may allow at least some components to be assembled before they are inserted into the housing. In addition, components close to the bottom of the housing, for example, the line straps, may be inserted more easily if the circuit breaker housing has open space through which these components may be guided to their destinations.
The insertion of the line contact arms or line straps into a circuit breaker housing may be particularly difficult because these components often have shapes which bend back upon themselves. The line contact arms are shaped so that the line and load terminals may be on opposite sides of the breaker but also, for blow-open operation, so that at least a portion of the line contact arm may be parallel to a corresponding portion of the load contact arm. For the blow-open mechanism to work properly, the current flow through the parallel portion of the load contact arm should be in the opposite direction to the current flow through the corresponding parallel portion of the line strap. These line straps are typically in the bottom of the breaker housing and include a line terminal which is accessible from outside the circuit breaker housing. Thus, in many existing breakers, the line straps are dropped into the bottom of the housing and then maneuvered to push the line terminal through a slot in the side of the circuit breaker.
In conventional circuit breakers, there is often a tradeoff between ease of assembly and structural integrity. If a circuit breaker housing is designed with too much open space, it may not be strong enough structurally to withstand the normal forces to which the circuit breaker is subject to during installation and use, especially forces generated when the breaker is subject to heavy short-circuit currents. During a heavy short-circuit condition, electrical arcing which occurs when the line contact is separated from the load contact may cause an explosive rise in pressure inside the breaker housing.